Investigating the Genetic Basis of Type 3 of Von Willebrand Disease (VWD)
bleeding disorder , von Willebrand factor , von Willebrand disease
von Willebrand Disease (VWD) is the most common inherited bleeding disorder in humans, resulting from quantitative or qualitative deficiencies of von Willebrand factor (VWF). Type 3 VWD is the rarest and most severe form of the disease. This thesis characterizes the phenotype-genotype correlations of a cohort of Canadian type 3 VWD patients and their family members. Three main findings are highlighted: 1) 50% of families showed evidence of co-dominant inheritance as opposed to recessive, 2) 42% of mutations identified were located in the VWF propeptide region (VWFpp), 3) index cases (IC) with mutations in the VWFpp had a more severe bleeding diatheses than IC with mutations elsewhere. We investigated two of the identified VWFpp mutations (ex4-5del and Cys633Arg) to elucidate their molecular mechanisms using two cellular models. Patient-derived blood outgrowth endothelial cells (BOEC) are ideal for studying the underlying molecular mechanism of VWF mutations as they represent the native vascular endothelium. BOEC were isolated from type 3 VWD IC and family members with the mutations of interest. A heterologous cellular system was also used to study the VWF mutations in vitro. The VWFpp mutations caused impaired VWF secretion, defective multimerization, qualitative and quantitative defects in Weibel-Palade body (WPB) formation, and resulted in VWF retention within the endoplasmic reticulum. We attempted to restore secretion and multimerization by co-transfecting each mutant with the wild-type VWF propeptide (VWFpp), which was unsuccessful. Additionally, we investigated a third mutation, c.8419_8422dupTCCC, which is unique to the Canadian VWD population and is found at a high frequency in a specific geographic population. While we hypothesized that this mutation would disrupt dimerization due to its location in the C-terminal cysteine knot (CK) domain of VWF we did not find this to be true. The results presented within this thesis provide new insight into the genetics and pathobiology of type 3 VWD, the functional contribution of the VWFpp to type 3 VWD and highlight the utility of BOEC as a cellular model for evaluating the pathogenic mechanisms of VWF mutations.